In recent years, the contactless power transmission technology is attracting attentions, with which the power is transmitted from a power-transmitting side to a power-receiving side in a non-contact manner. For example, a power transmission system using the contactless power transmission technology has been disclosed in Patent Document 1. In this power transmission system, when the positional relationship between the power-transmitting side and the power-receiving side is changed, i.e., when the coupling coefficient between the coil on the power-transmitting side (the antenna on the power-transmitting side) and the coil on the power-receiving side (the antenna on the power-receiving side) is changed, the operation of a DCDC converter (a Buck-Boost portion) connected to the coil on the power-receiving side (the antenna on the power-receiving side) via a rectifier portion is controlled so that the charging efficiency of a battery is improved. Thus, if the operation of the DCDC converter (the Buck-Boost portion) is controlled, the optimum charging voltage can be supplied to the battery even if the voltage output from the rectifier portion is changed.